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This invention relates to an optical spectral coding scheme for a high-performance optical code-division multiple-access (OCDMA) network system. The encoder and decoder are structured with cascaded fiber Bragg grating (FBG) devices. The scheme can eliminate the multiple-access interference (MAI), and can promote the CDMA system capacity. Using the simple coder structure and low cost devices, the invention can be used on switching routers to connect local network computers, or on exchange modules for signal switching between network nodes. It is applicable to Asymmetric Digital Subscriber Loop (ADSL), or Cable Modem to connect with digital home network, local area network (LAN), and Internet etc.
Optical networking is one way to provide a range of telecommunications services to meet the growing demands of an information-based society. However, existing multiple access implementations for LAN (local-area network) or MAN (metropolitan-area network) networks are apparently inadequate. Optical code-division multiple-access (OCDMA) offers versatile connection between numerous local network users, along with generous control on the wavelength stability or network synchronization requirements. Hence OCDMA techniques have drawn much attention in recent years.
Fiber Bragg gratings (FBG) are produced by exposure of photosensitive fiber to ultraviolet light. They have a refractive index that is spatially periodic along the propagation axis of a fiber. The desirable filtering characteristics of fiber Bragg gratings are disclosed by K. O. Hill and G. Meltz (IEEE J. Lightwave Technology, vol. 15 (8), pp. 1263-1276, 1997) and have been employed in some optical devices. For example, the Distributed Bragg Reflector (DBR) is designed with Bragg gratings in the laser resonant cavity. The DBR accumulates energy in the resonant cavity and emits light when the accumulated energy reaches a threshold. Fiber gratings have also been used for performing optical measurements. For example, due to the Bragg wavelength""s change with tension or temperature, fiber Bragg gratings can measure the variation of external factors. In addition, the gratings can be used for the gain compensation of an Erbium-Doped Fiber Amplifier (EDFA), for the analyses improvement of spectral analyzer, and so on.
In optical fiber communications, fiber Bragg gratings can be employed as chromatic compensators. The propagation of long-wavelengths is slower than the propagation of short-wavelengths, causing the phenomenon of pulse broadening (or xe2x80x9cdispersionxe2x80x9d) in fiber transmission. This pulse broadening tends to reduce the data bit rate of digital communication. Fiber Bragg gratings provide xe2x80x9cchirped apodizationxe2x80x9d by reflecting long-wavelength components at the front end of the grating and short-wavelength components at the rear end of the grating. This compensates for chromatic dispersion by ensuring that the full light band spends the same amount of time to pass through chromatic dispersion and grating compensation. Another popular application of FBG is the realization of light wave filter with the desired wavelengths on the reflective end. Such characteristics of fiber Bragg gratings are utilized in this invention to construct a new scheme of optical CDMA encoder/decoder devices.
J. A. Salehi et al. disclosed the technology of code-division multiple-access (CDMA) [IEEE J. Lightwave Technology, vol. 8 (3), pp. 478-491, 1990] for the application of optical fiber communications. In the early periods, bipolar codes with good correlation properties, such as M-sequence or Gold code, were adopted for the optical CDMA communications. At the transmitter end, data bits are encoded into unipolar optical signals. The receiver makes an Electrical/Optical (E/O) conversion and has a bipolar address decoding procedure in the electrical domain. Since after E/O conversions, the receiver must go through the procedure of Sequence Inverse Keying (SIK), the system is named as the SIK-CDMA. The SIK-CDMA system needs multiple electrical/optical and optical/electrical conversions and hence has serious limitation on the data transmission rate. Thereafter, research efforts have aimed at the all-optical signal processing to promote the data transmission rate.
In the light wave domain, the optical signal is inherently a unipolar system. As stated by F. R. K. Chung et al. [IEEE Trans. on Inform. Theory, vol. 35 (3), pp. 595-604, 1989], unipolar signature code such as optical orthogonal code (OOC) and modified prime code (MPC) can have the same correlation characteristics as those of the traditional bipolar sequences. Since the cross-correlation of unipolar codes is incapable of achieving complete orthogonality, the number of xe2x80x9c1xe2x80x9d in code sequences must be restricted to improve the correlation characteristic. Unfortunately, this means that a given code sequence length should have relatively few xe2x80x9c1xe2x80x9d. The number of unipolar code sequences is therefore far less than that of the traditional bipolar sequence codes.
One of the early demonstrations of OCDMA uses optical delay lines and optical orthogonal codes for OCDMA time domain coding. The time-encoded optical CDMA coder is as presented in FIG. 4. In this delay line configuration of FIG. 4, the incoming signal is split into several independent paths in which each signal is delayed according to the specific delay elements of the desired optical orthogonal codes. The tapped delay line scheme suffers from high splitting loss due to intensity splitting among the optical delay lines. Also, to comply with the hasty growth on the number of users, one needs to substantially lengthen the code sequence to promote the system capacity. This increases system expenditures and is unsuited for the economical benefit.
On retracing the past technology, there were cases on utilizing the concept of optical phase coding to implement code-division multiplexing. These technologies need coherent, ultrashort pulses, with transmitter and receiver being wavelength and phase coherent. L. R. Chen et al. investigated ultrashort pulse propagation in fiber Bragg gratings [IEEE Journal of Quantum Electronics, vol. 34 (11), pp. 2117-2129, 1998] with applications on Wavelength Division Multiplexing (WDM) and Code-Division Multiple-Access (CDMA) systems. The adopted incident optical source is broadband ultrashort pulse. The coding scheme is the table-lookup type of frequency hopping. The decoder grating was arranged in a reverse order to that of the encoder grating to accomplish the same optical path for every component spectral chip.
A well-known frequency encoder for optical broadband sources is disclosed in the article by M. Kavehrad and D. Zaccarin [IEEE J. Lightwave Technology, vol. 13 (3), pp. 534-545, 1995], and is shown in FIG. 5. This is the typical representative of incoherent broadband CDMA systems. The optical frequency coder of FIG. 5 consists of a pair of diffraction gratings placed at the focal planes of a unit magnification confocal lens pair. The first grating spatially decomposes the spectral components present in the incoming optical signal with a certain resolution. A spatially patterned mask is inserted midway between the lenses at the point where the optical spectral components experience maximal spatial separation. After the mask, the spectral components are re-assembled by the second lens and second grating into a single optical beam. The mask can modify the frequency components in phase and/or in amplitude, depending on the coherence property of the incident optical source. The apparatus has been used with high-efficiency for temporal shaping of short pulses. An example has been illustrated by R. A. Griffin et al. [IEEE J. Lightwave Technology, vol. 13 (9), pp. 1826-1837, 1995] in an optical coherence coding scheme to implement optical frequency hopping CDMA. This article is the typical representative of coherent ultrashort pulse CDMA systems.
Japan Pat. No. JP9312619A issued to Thomas Pfeiffer in 1997 brings up another frequency-coding scheme of optical CDMA transmission system. By virtue of a Fabry-Perot-like periodic optical filter, the LED or Er-ASE source spectrum is divided into several narrow optical pulses, and then processed with Electrical/Optical (E/O) data modulation. At the receiver, an optical coupler makes Optical/Electrical (O/E) conversions on the received accumulated optical pulses. The signals are then distributed to different bandpass filters by a branching device. Each receiver has a local oscillator to produce the necessary signals for data demodulation. The electrical signal finally passes through the lowpass filter to retrieve the desired data information. The encoding mechanisms in this patent periodically sliced the optical source and then proceeded with the optical modulation to achieve the coding effect.
A. Grunnet-Jepsen et al. incorporated phase shifts and wavelength chirps among the grating segments and demonstrated coherent spectral phase coding of pulses for use in CDMA systems [OFC/IOOC""99 conference proceeding, pp. PD33/1-PD33/3, 1999]. X. Wang et al. used prime codes over FBGs to demonstrate experimentally a novel hybrid temporal/spectral coding technique for fiber optic CDMA application [OFC/IOOC""99 conference proceeding, pp. PD34/1-PD34/3, 1999]. Both authors adopted ultrashort pulses with coherent spectral coding scheme for simultaneous optical pulses operation on the time and frequency domains.
R. Papannareddy and A. M. Weiner [IEEE Photonics Technology Letters, vol. 11 (12), pp. 1683-1685, 1999] evaluated performance comparisons between coherent ultrashort pulse and incoherent broadband CDMA systems. Though ultrashort pulse CDMA can in principle yield a substantial throughput advantage over the incoherent broadband systems, incoherent threshold energy detection is believed to be a more reliable approach than coherent grating pulse alignment. Moreover, the scheme of coherent OCDMA needs femto-second ultrashort pulse technology. This is still a great challenge at the present time.
This invention discloses a xe2x80x9cFiber Bragg Grating-based Optical CDMA Encoder/Decoderxe2x80x9d scheme. It utilizes the fine filtering function of fiber Bragg gratings to produce reflected and transmitted narrowband spectral chips with specifically designed wavelengths. The invention combines the precise filtering characteristics of optical fiber grating and the pseudo-orthogonal correlations of maximal-length sequence codes (M-sequence codes) to structure encoder/decoder modules for a fiber-optic CDMA system.
The technology of optical CDMA allows multiple users in local area networks (LAN""s) to access the same fiber channel asynchronously with no delay or scheduling. Multiple users within the same channel can simultaneously deliver data messages and share the same wideband resources. The technique of fiber-optic CDMA is a very good selection for developing optical fiber as a broadband transmission medium. Code-division multiple-access system possesses anti-interference characteristics and can solve the bursty traffic problem. It enhances the system capacity and has good secrecy. Hence it is proving desirable for wireless and fiber-optic communications.
To solve the deficiency that only unipolar sequence can be used on the conventional optical CDMA system, this invention aims at using fiber Bragg gratings to generate specific wavelength spectral chips. With the pre-written M-sequence codes in the fiber gratings, the invention devises a simple scheme for the fiber-optic CDMA encoder/decoder. With such a scheme, traditional M-sequence codes can be suitably applied for the optical fiber communications. The scheme can solve the deficiency that unipolar sequence has to be largely lengthened to promote the number of system users. More importantly, on utilizing the quasi-orthogonal correlations of M-sequence codes, the invention can eliminate the multiple-access interference (MAI) caused by other users in the system. This greatly enhances the system performance and reduces the transmission error probability.
On judging system performance under the same bandwidth and bit error probability, coherent ultrashort pulse CDMA can in principle yield substantial throughput advantage over the incoherent broadband CDMA. However, coherent CDMA needs femto-second ultrashort pulse technology. This is still a great challenge at the present time. This invention may be used in non-coherent optical CDMA systems. The invention combines the on-off keying modulation of broadband LED or Er-ASE sources and the pseudo-orthogonal characteristic of M-sequence codes written in the fiber gratings. The scheme can eliminate multiple-access interference (MAI) encountered in optical CDMA systems. Unlike traditional OCDMA systems, the present invention avoids the use of diffraction gratings, confocal lenses, and/or optical phase masks. Instead, the invention devises properly coded fiber grating devices to simplify system complexity The reliability of fiber Bragg gratings is also dependable.